Bird Networks in the Amazon (complexity)

Relationships between organisms are broad

  • Competition
    • today


  • Mutualisms
    • jigsaw


  • Predation and Consumption
    • next week


  • Parasites and hosts


  • Humans vs other species
    • friday data exercise

What is competition?

WHO: Intra vs Inter-specific competition

Competition always reduces the fitness of both parties

Intraspecific competition limits population growth

Competition is a powerful driver of evolution




  • Natural selection favors individuals that do not compete


  • Drives niche differentiation, trait specialization and potentially species diversification


  • Directional selection can occur causing character displacement

Gause’s Law: Theory of Competitive Exclusion





  • Two species cannot occupy the exact same niche if they need the same resource


  • Those species cannot coexist at stable constant populations
    • a winner and loser will emerge


  • Leads to local extinction of weaker competitor or shift to a different ecological niche

Gause’s test tube Paramecium study

Red vs Grey Squirrel in Britain (Murphy 2015)

Competitive exclusion is harder to detect in nature

Competitive exclusion, however, is rare in nature



  • Resources are variable/limiting in an environment
    • leads to more symmetric competition (both struggle)


  • Species compete for multiple resources
    • exclusion based on a single resource is too simple


  • Complete niche overlap (test tube) is rare


  • TAKE HOME: Greater overlap = greater competition

Competition does result in the ‘Realized Niche’

Joseph Connel: Competition among barnacles

Can’t we all just get along?

Co-existence and natural selection

  • Overlapping species may alter resource use to minimize competition
    • resource partitioning = division of resources


  • Species whose niches only partly overlap coexist easier
    • use the environment differently = niche differentiation
    • evolve traits to use different/less overlapping resources = character displacement


  • Co-existing species can overlap by using different forms of resources

Co-existence through resource partitioning in lizards

Niche differentiation: different use of microhabitats

Here, different insects forage for resources at different locations and times

Character displacement in cicada mating calls